The RNA binding properties of ‘native’ protein—protein complexes isolated from the Escherichia coli ribosome

The assembly of the Escherichia coli ribosome is based on many specific interactions between the ribosomal components. Direct evidence for single proteinRNA interactions has been well established (reviewed in ref. [l] ). Various observations provide circumstantial support for the existence of protein-protein complexes in the ribosome. For example, cooperative protein binding effects have been observed during 30 S subunit reconstitution [2] and in protein assembly to 5 S and 23 S RNA [3,4]. Moreover, many protein pairs have been cross-linked with relatively short reagents [5,6], one pair of which was subsequently shown to be active in reconstitution experiments [7]. The first direct evidence, however, for the occurrence of a specific protein-protein interaction came from the isolation of a very stable complex between proteins L7/12 and LlO from the 50 S subunit [8]. Conventional purification procedures for ribosomal proteins purposely use strong dissociating conditions [9], which would disrupt any native protein compiex. Isolation of ribosomal proteins under nondenaturing conditions, however, has made it possible to purify several putative ‘native’ protein complexes from both ribosomal subunits [IO] . This paper deals with two of these potential complexes, namely S13S19 and L7/12-LlO. They bound specifically to 16 S and 23 S RNA, respectively, whereas their constituent proteins showed little (S13 and S19) or noti (L7/12 and LIO) binding to RNA. We consider that the specific RNA binding properties of these two protein complexes provide direct evidence that they are biologically significant complexes.